ABCD4 is a lysosomal ATP-binding cassette transporter essential for cobalamin (vitamin B12) homeostasis 1. This protein functions as an ATP-dependent transporter that mediates cobalamin export from the lysosomal lumen to the cytosol 2, a process mechanistically dependent on its ATPase activity 2. ABCD4 localizes to lysosomal membranes through recruitment by the chaperone protein LMBRD1 from the endoplasmic reticulum 3. At the lysosomal membrane, ABCD4 forms a functional complex with LMBRD1 and the cytosolic protein MMACHC, which facilitates vectorial cobalamin delivery to prevent cofactor dilution and protect against inactivating reactions 4. Clinically, ABCD4 mutations cause methylmalonic aciduria and homocystinuria type cblJ (cblJ deficiency), a cobalamin metabolism disorder characterized by impaired cobalamin cofactor synthesis 3. Patients present with metabolic dysfunction affecting methionine synthase and methylmalonyl-CoA mutase, the two human cobalamin-dependent enzymes 5. Disease-causing mutations disrupt either the ATPase domain or clinical domain of ABCD4, impairing its interaction with LMBRD1 and subsequent lysosomal targeting 3. Animal models demonstrate that ABCD4 loss results in vitamin B12 deficiency-associated anemia with abnormal red blood cell development 6.